It is proposed to continue with the investigation of bacteriophage P4, a satellite virus, and of the interactions between P4 and genomes of P2-group phages, which act as helpers for P4, further studies in the following areas: (1) the mechanism of transactivation by P4 of helper prophages and the involvement of the early gene region of the helper genome (in particular by employing transactivation refractory hybrids between P2 and P2-related phages as well as transactivation refractory P2 mutants); (2) the nature of a bacterial mutation (equal gro) which interferes with the growth of P2 (and P4) and the nature of a P2 mutation (equal ogr) enabling P2 to overcome the gro block (also with respect to assisting P4); (3) the mechanism of prophage induction in the P2-P4 system and especially the role of the cox function(s) of P2; (4) analysis of new P4 mutants and mapping the P4 chromosome; (5) the morphogenetic function(s) of (late) P2 genes with mutations interfering more severely with P4 than with P2 development; (6) the mechanism of fertility reduction of male E. coli cells by prophage P4, using Hfr and F' males and also R factor carrying strains.